1. Field of the Invention
The present invention relates to a backlight unit in which a plurality of optical sheets are to be placed, a liquid crystal display apparatus equipped with such a backlight unit and a, method of assembling the backlight unit by a plurality of optical sheets.
2. Description of the Related Art
A liquid crystal display apparatus has been widely used as a monitor apparatus for an office automation (OA) apparatus, television and others because of its characteristics such as compactness, thinness and low-power consumption. The liquid crystal display apparatus is mainly composed of a liquid crystal panel in which liquid crystal held between opposing transparent substrates and a backlight unit for radiating backlight to illuminate the liquid crystal panel.
The backlight unit is primarily composed of a light source for radiating illuminating light (so-called “backlight”), a light guide plate for guiding the illuminating light equally, several kinds of optical sheets such as a diffusion sheet, lens sheet and polarizing sheet, a reflection sheet for reflecting the illuminating light leaking out to the rear side toward a liquid crystal panel and a chassis for holding these components.
For an optical sheet used in a backlight unit, serializing order, the front and back thereof, and up/down and left/right directions are predetermined for assembling into a chassis. However, every optical sheet is very similar in appearance and difficult to distinguish between front and rear faces, so that it is liable to be placed in a wrong order and/or a wrong direction. If the optical sheet is assembled in an incorrect order or an incorrect direction, it causes a defect that luminance of the backlight unit is lowered or luminance is not evenly distributed.
Therefore, in a conventional method for assembling the backlight unit, plural optical sheets have been sequentially superposed while checking functions of each of optical sheets by referring to specifications. If different optical sheets are mixed, it is difficult to discriminate them, so that the optical sheets are kept for each of the types in trays and bags on which each of names of the optical sheets are written to readily discriminate the types. Even in working for superposing the optical sheets, they are classified using trays on which the names of respective optical sheets are written for preventing the optical sheets from being mixed.
Japanese Patent Laid-Open Nos. 2000-11728 and 2004-333520 disclose optical sheets with a structure which can place each optical sheet in an appropriate order and direction.
The optical sheet disclosed in Japanese Patent Laid-Open No. 2000-11728 has a structure with a lug part for each optical sheet. A position of the lug part for each optical sheet is shifted from one another among a set of optical sheets, and a part of the lug part of one optical sheet is sequentially superposed on the lug part of the other optical sheet. The optical sheet disclosed in Japanese Patent Laid-Open No. 2004-333520 has a structure with a notch for each optical sheet. The notch is provided on the periphery of the optical sheet, and a position of the notch for each optical sheet is shifted from one another among a set of optical sheets, and the notches are different from each other in size and shape.
The optical sheets disclosed in Japanese Patent Laid-Open Nos. 2000-11728 and 2004-333520 are so structured as to sequentially shift positions where the lug parts and the notches of the optical sheets are provided. That is to say, it is structurally intended that the optical sheets continuously placed upward and downward are sequentially associated with each other.
For this reason, when a part of the optical sheets should be removed due to revision of specifications of an apparatus, for example, the optical sheets those which have been placed on and beneath the optical sheet to be removed can not be associated with each other any more. As a result, if a part of the optical sheets is removed, all the optical sheets placed on and over the removed optical sheet should be replaced by new optical sheets in which the positions of lug parts and the notches are also modified to keep sequential association with each other among rest of the optical sheets, which causes a problem in that it takes much time and labor and cost is increased.
This problem is described with reference to FIGS. 24A and 24B and FIGS. 25A and 25B.
FIG. 24A is a plan view schematically showing the state where lug parts 16a to 16d so formed as to shift in position from each other and as to partially and sequentially superposed on each other are provided on the four optical sheets 15a to 15d. FIG. 24B is a plan view schematically showing the state where all of the four optical sheets 15a to 15d are superposed. That is to say, when all of the four optical sheets 15a to 15d are superposed, as is clear from FIG. 24B, the lug parts 16a to 16d are partially and continuously superposed.
FIG. 25A is a plan view schematically showing the state where out of the above four-layer optical sheets 15a to 15d the second-layer optical sheet 15b is removed. FIG. 25B is a plan view schematically showing the state where the other three optical sheets 15a, 15c and 15d left after the second optical sheet 15b has been removed are superposed As shown in FIG. 25B, in the structure of a conventional optical sheet, if the second optical sheet 15b has been removed, the lug part 16a of the bottom-layer optical sheet 15a is separated from the lug parts 16c and 16d of the two optical sheets 15c and 15d which are placed over the bottom layer. For this reason, the two optical sheets 15c and 15d need to be replaced by new optical sheets in which the respective positions of lug parts 16c and 16d are modified to keep sequential association with each other among the optical sheets 15a, 15c and 15d. 
In addition, in the structure of a conventional optical sheet, when a part of the optical sheets should be replaced by another new optical sheet due to revision of specifications of the apparatus, the old optical sheet to be replaced and the new replacing optical sheet should be the same in shape to keep the current association among all optical sheets (in other words, the association between the old optical sheet to be replaced and rest of the remaining optical sheets should also be kept in the association between the new replacing optical sheet and rest of the remaining optical sheets). That is to say, the lug part and the notch formed on the new replacing optical sheet should be the same in shape and place as those formed on the old optical sheet to be replaced. For this reason, it is difficult to distinguish between the old optical sheet to be replaced and the new replacing optical sheet from appearance, which causes confusion in which the optical sheets before and after replacement are mixed up.
The lug part and the notch provided on the conventional optical sheet merely show the order and direction in which each optical sheet is placed, but not showing information related to the optical sheet. Therefore, in order to obtain information on the optical sheet such as functions, manufacturer's name, revision number, thickness and others, it is necessary to refer to specifications each time, which causes a problem in that work efficiency is lowered.
The present invention has been made in view of the above problems. Objects of the present invention are to provide a backlight unit, a liquid crystal display apparatus and a method of assembling the backlight unit in which a large number of optical sheets can be simply and surely assembled into a chassis in an appropriate order and direction, to provide the same which allow flexibly coping with a change in structure of the optical sheet, and also to provide the same in which information of each optical sheet can be easily obtained to improve work efficiency.